1. Field of the Invention
The present invention relates to a toner for developing a latent electrostatic image, which controls the dispersion state of wax near the toner surface to suppress sticking and be excellent in fixing property, and the present invention also relates to a method for producing the toner, an image-forming apparatus and a process cartridge using the toner.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a toner image is formed on a photoconductor by the following steps: charging the surface of the photoconductor which is an image bearing member by discharging, exposing the charged surface of the photoconductor to form a latent electrostatic image, developing the latent electrostatic image by supplying a toner having an opposite polarity to the latent electrostatic image which is formed on the surface of the photoconductor. The toner image formed on the photoconductor is transferred to an intermediate transferring medium, and then transferred from the intermediate transferring medium to a recording member such as a paper, or otherwise, transferred directly from the photoconductor to the recording member. Then, the transferred toner image is fixed on the recording member by heating and pressing.
In the above steps, the recording member is sandwiched between a pair of roller- or belt-shaped fixing members each having a heater inside thereof so as to fix the toner image on the recording member by heat-melting and pressing the toner. When the heating temperature is too high, the toner melts excessively and the toner is fused on the fixing member, namely hot offset is occurred. When the heating temperature is too low, the toner does not melt enough, and the fixing property is insufficient. From the viewpoint of saving energy and downsizing an image forming apparatus, the toner having higher hot-offset generating temperature, or hot-offset resistance, and lower fixing temperature, or low-temperature fixing property are required.
Particularly, in a full-color copier and a full-color printer, a toner has to have a low-melting viscosity, as the glossiness and color mixing property of an image is required. Thus, a polyester toner binder having sharp melt property is used. Because hot offset is likely to be occurred in this kind of a toner, a silicone-oil or the like is coated on the fixing member in the conventional full-color apparatus. However, the configuration of such the apparatus is complicated and the size thereof becomes large, because an oil tank, and a device for coating oil are required in order to coat the silicone oil on the fixing member. In addition, maintenance is needed periodically due to the degradation of the fixing member. Moreover, it is unavoidable that the oil adheres to a copy paper, a film for OHP, i.e. Overhead Projector, or the like. Especially when a printing is performed on an OHP film, the color tone may be degraded by the oil adhesion.
Therefore, the method of adding a wax to a toner is generally used to prevent adhesion of the toner without coating oil on the fixing member. However, the effect of releasing ability is greatly influenced by the dispersion state of the wax in the toner binder.
Japanese Patent (JP-B) No. 2663016 discloses that a low melting wax which cannot be used in a pulverization toner is contained in a toner which is produced by suspending and polymerizing a substance having a polar group and a polymerizable monomer having a releasing agent in water. The nonpolar component such as a wax does not exist near the surface of toner particle on the contrary to the polar component, and forms a pseudocapsule structure which is covered with the polar component forming the surface. However, in this technology, the wax is strongly retained inside of the toner, so that wax exudation after heating is inhibited. Thus, the wax insufficiently exudes, and then the releasing property is not sufficiently exhibited.
Japanese Patent Application Laid-Open (JP-A) No. 2002-6541 discloses that a toner consisting of toner particles each including a wax inside, and the wax locally exists near the surface of the toner particle. However, in this technology, the amount of the wax based on the total amount of the toner is not described. In only one example described therein, the amount of the wax based on the total amount of the toner particles is less than 1%. The wax exudes on the surface of a fixing member to prevent the toner adhesion thereto when fixing, however, the releasing effect by the wax is insufficient due to the small amount of the wax, and then the hot-offset margin may be lost.
JP-A No. 2004-246345 discloses that the dispersion state of the wax near the toner surface is controlled to improve the hot-offset resistance, thereby producing a toner having excellent fixing property, and to improve blocking resistance in an electrophotographic image forming, thereby having excellent storage property for a long time. The amount of the wax in the toner is determined by FTIR-ATR, i.e. Fourier Transform Infra-Red attenuated total reflection Spectroscopy. However, the amount of the wax on the surface of the toner after heating is not described.
JP-A No. 2004-258625 discloses that an image forming method using the combination of a developer, in which the dynamic friction coefficient of the toner influenced by the wax on the toner surface is adjusted to minimize the biased abrasion in a photoconductor over time, and a process which is optimal for the developer in an image forming system which has been achieved lower cost, and smaller in footprint. JP-A No. 2004-258625 also discloses the toner containing at least a binder resin and a wax, and having the dynamic friction coefficient of 0.15 to 0.45. However, the dispersion state of the wax near the toner surface is not described therein.